MA0042: add DbCommand await-using parity with DbConnection

[meziantou]

MA0042 already avoids reporting using -> await using for directly instantiated DbConnection subclasses when they do not override DisposeAsync, because the base implementation is synchronous. This change applies the same behavior to DbCommand to avoid unnecessary diagnostics in equivalent cases.

What changed

• Extended DoNotUseBlockingCallInAsyncContextAnalyzer to resolve System.Data.Common.DbCommand and apply the same direct-new subclass check used for DbConnection.
• For DbCommand instances created with new, diagnostics are now reported only when DisposeAsync is declared in the subclass chain (excluding DbCommand itself).
• Kept existing behavior for factory-returned DbCommand instances, which still report because runtime types may override DisposeAsync.
• Added MA0042 tests covering:
  • direct new with no override (no diagnostic)
  • factory method with no override (diagnostic)
  • direct new with override (diagnostic)
  • override in an intermediate base (diagnostic)
• Updated docs/Rules/MA0042.md to document the new DbCommand behavior alongside Stream and DbConnection.

Validation

• Ran DoNotUseBlockingCallInAsyncContextAnalyzer_AsyncContextTests on default Roslyn.
• Ran the same test set with /p:RoslynVersion=roslyn4.2.
• Ran dotnet run --project src/DocumentationGenerator until stable (no further markdown updates).

Contributes to #1121